Re: O'Hare Sighting Cloud Hole Effect - Shough

From: Martin Shough <parcellular.nul>
Date: Thu, 11 Jan 2007 17:21:45 -0000
Fwd Date: Thu, 11 Jan 2007 19:29:11 -0500
Subject: Re: O'Hare Sighting Cloud Hole Effect - Shough
>From: Michael Tarbell <mtarbell.nul>>To: ufoupdates.nul>Date: Wed, 10 Jan 2007 14:02:37 -0700>Subject: Re: O'Hare Sighting Cloud Hole Effect
<snip>
>One might consider using specific wavelengths (e.g., microwaves)>to selectively heat the water rather than the entire air mass,>but I don't think this would substantially alter the energy>requirements: the droplets may become arbitrarily warm, but the>vast majority of the air around them is still saturated and will>not accept additional vapor. Even if the droplets were to "boil>off", the vapor thus generated would rapidly cool and recondense>in the surrounding air, until the air/vapor mixture finally>equilibrated above the dew point temperature. So the only>difference in this scenario is that the air is being heated by>interaction with hot water rather than directly. The energy>required should be virtually the same (but comments are>solicited here).
Hi Mike
I applaud your attempt to extract latent quantitative data from
this observation. We need to be confident that witnesses
described the nature and scale of the "hole" phenomenon
accurately (was it literally bored right through to the clear
sky for example?), and we need to get some feeling for the cloud
thickness based on rawinsonde humidity data. Hopefully these
things will be forthcoming.
As for the above paragraph: I'm not _too_ confident here, but
this is my take on it based on a kinetic point of view which I
think is physically fundamental. See:
http://www.ems.psu.edu/~fraser/Bad/BadClouds.html
Dalton's law states that gases in a vapour behave independently
on one another. The mean free paths of air molecules and water
molecules in water vapour are on average very long, and there's
plenty of room for more water molecules. So it seems to be a
misconception (a didactic fairy tale) that the air has a
literally saturated condition in which it will not accept more
water vapour.
Air does not "hold" water vapour at all, and if we were to
remove the air from the equation, the equilibrium vapour
pressure would continue to have exactly the same relation to
temperature as it does with the air present. Equilibrium vapour
pressure is entirely a matter of balance between evaporation and
condensation, i.e. it measures the average kinetic energy of
water molecules at the droplet boundary, and the condition of
the air is causally irrelevant, even though it happens that the
air temperature and the water temperature are related through
the same energetic process.
The temperature of the air obviously has an effect on the
temperature at the droplet surface, but this is not necessarily
net positive. If you heat the air then eventually this will
secondarily heat the droplets, but most of the heat energy in
the air will not be available at the droplet surfaces, only the
average heat energy per unit droplet area can be useful.
So I _think_ (!?) that putting the same energy selectively into
water droplets directly, i.e. by microwave tuned at the right
resonant wavelength, would evaporate cloud more efficiently than
by spreading the same energy in a broad band through the
droplets and the air mass.
Martin